This is the crown prince’s first major diplomatic test since the murder in Turkey of Saudi journalist Jamal Khashoggi, a key critic of the government. Saudi Arabia has blamed “rogue agents” but suspicion has fallen on the prince for ordering the killing.

After the photo was taken, the crown prince did manage a hearty exchange with Mr Putin.

French President Emmanuel Macron held a five-minute exchange with the crown prince during which, a French official said, Mr Macron had conveyed “very firm” messages over the Khashoggi affair.

Mr Trump had a “friendly meeting” with the crown prince, Saudi media said, while UK PM Theresa May has also said she will meet him.

Mrs May held a 15-minute meeting to discuss trade with Mr Macri. Their two nations have long disputed sovereignty of the Falkland Islands.

Mr Macri’s opening address to the summit admitted there would be issues of disagreement that needed to be managed.

He called for consensus on sustainable development, the future of work, food supply, climate change and international trade, adding: “I am an optimist”.

What will go on on the sidelines?

There are certain to be plenty of bilateral, trilateral or even larger get-togethers, although the one that will not be taking place is the highest-profile.

Mr Putin’s spokesman, Dmitry Peskov, said Moscow regretted the decision. But he said: “If this is so, the president will have a couple of extra hours in the programme for useful meetings on the sidelines of the summit.”

German Chancellor Angela Merkel has blamed the crisis “entirely” on Russia and said she would raise the issue with Mr Putin.

The second-highest profile meeting will also involve Mr Trump. He will have dinner on Saturday with his Chinese counterpart, Xi Jinping.

The key issue there will be the nations’ trade tariffs dispute.

The US has hit a total of $250bn (£196bn) of Chinese goods with tariffs since July, and China has retaliated by imposing duties on $110bn of US products.

China said consensus was increasing on a deal but that differences remained.

After earlier indicating pessimism on an agreement, Mr Trump said: “They want to and we’d like to. There are some good signs. We’ll see what happens.”

Mr Putin has also weighed in, with a thinly veiled attack on Mr Trump’s America First policy. The Russian president condemned “dishonest competition” along with “vicious” sanctions and protectionist measures.

The Brics group, comprising Brazil, Russia, India, China and South Africa, issued a statement saying protectionism ran counter to the “spirit and rules of the World Trade Organization”.

Will there be a joint G20 communiqué?

European diplomats told Associated Press there has been tough haggling over a final statement.

Argentine officials indicated differences on trade could be overcome.

However, that will still leave the obstacle of climate change.

Mr Trump has not hidden his objection to collective action on the issue.

UN chief Antonio Guterres said in Buenos Aires that “this is a make-it-or-break-it moment” on the issue.

French President Emmanuel Macron was quoted by AFP news agency as saying he would refuse to sign a trade deal with South America’s Mercosur bloc if Brazilian President-elect Jair Bolsonaro withdrew from the Paris climate accord.